Electoacoustic transducers and electromagnetic assembly therefor

ABSTRACT

An electroacoustic transducer comprising a diaphragm and an electromagnetic assembly in driving relationship with the diaphragm, the assembly comprising a magnet, a core defining a gap in the magnet, a coil supported by a coil holder movably arranged in the gap with an end portion facing the diaphragm, a cap on said end portion having a top surface directed toward, and in driving relationship with, the diaphragm, the surface being preferably secured to the diaphragm by means of an adhesive. The adhesive may be soft or hard and the diaphragm is preferably flat. The cap seals the coil holder and a perforation is provided connecting the space formed between the cap, coil holder, core and gap with another surrounding whereby a rectilinear to and fro movement pump is defined by the cap, coil holder and core, the pumping action moving air to cool the coil. The lower end of the coil holder has a flange to prevent the coil from being shockremoved from the coil holder, and the upper end of the holder has a flange which is adhered to the cap.

United States'Patent [1 1 Bertagni I541 ELEQTROACOUSTIC TRANSDUCERS ANELECTROMAGNETIC ASSEMBLY THEREFOR [76] Inventor: Jose Juan Bertagni,1027 [57] 22 Filed:

211 Appl. No.: 259,378

[30] Foreign Application Priority Data Hernandarias St., Buenos Aires,Argentina June 5, 1972 June 16, 1972 Argentina 236220 [52] U.S. Cl.335/231, 179/115.5 [51] Int. Cl. 1101f 7/00 [58] Field of Search..335/23l; 179/1155 R,

[56] References Cited UNITED STATES PATENTS 2,252,846 8/1941 Giannini etal. 179/1 15.5 R 2,501,032 3/1950 Harbaugh 179/1155 PC 2,848,561 8/1958Gorike l79/ll5;5 R 3,329,777 7/1967 Kliewera. 179/1155 R 3,567,8703/1971 Rivera 179/l15.5 R to the cap- 37 Claims, 8 Drawing FiguresPrimary Examiner-George Harris Attorney, Agent, or FirmMilton OsheroffABSTRACT An electroacoustic transducer comprising a diaphragm and anelectromagnetic assembly in driving relationship with the diaphragm, theassembly comprising a magnet, a core defining a gap in the magnet, acoil supported by a coil holder movably arranged in the gap with an endportion facing the diaphragm, a cap on said end portion having a'topsurface directed toward, and in driving relationship with, thediaphragm, the surface being preferably secured to the diaphragm bymeans of an adhesive. The adhesive may be soft or hard and the diaphragmis preferably flat. The cap seals the coil holder and a perforation is Iprovided connecting the space formed between the 179]! BS cap, coilholder, core and gap with another surrounding whereby a rectilinear toand fro movement pump is defined by the cap, coil holder and core, thepumping action moving air to cool the coil. The lower end of the coilholder has a flange to prevent the coil from being shock-removed fromthe coil holder, and the upper end of the holder has a flange which 56 w4 1 flw Apr. 2, 1974 ELECTOACOUSTIC TRANSDUCERS AND ELECTROMAGNETICASSEMBLY THEREFOR The present invention relates to an electromagneticassembly for electroacoustic transducers and the like, and moreparticularly it refers to an electromagnetic assembly, as well as anelectroacoustic transducer utilizing said assembly, which include, dueto its structural and constitutive arrangement, a number of particularfeatures, each of which may be used alone or in combination to achievedifferent effects and results. Thus, one of the features is to use thevibratory movement of the coil holder to act as a plunger of a to andfro moving pump in order to cool or refrigerate the core of the magnet,i.e. the pole piece of the magnet, the gap, the

coil holder and the coil or to replace the'elastic spider whichresiliently supports the coil holder. The coil may also be mounted onthe coil holder in a particular way in order to reduce the distributedcapacity. The electromagnetic assembly of the present invention may alsobe used below the water, for example in connection with loudspeakers andmicrophones. As to the aspect of using the electromagnetic assemblyunder the water, said assembly may be combined with vibratory panels forother purposes, such asfor a washing machine. It is also possible to usethe electromagnetic assembly in connection with vibratory panels totransmit such vibrations to granular materials arranged on saidvibratory panel.

As may be appreciated from the foregoing, the electromagnetic assemblyof the present invention is applicable to various types ofelectroacoustic transducers usable in a number of different fieldswithin industry, the assembly being in driving relationship with thediaphragm of the transducer. In "-other words, the diaphragm candrive'the electromagnetic assembly or the electromagnetic assembly candrive the diaphragm. The acoustic vibrations may be those which can beor are to be heard (e.g., music, speech, etc.), or those whose purposeis mere vibration (e.g., washing machine).

Another feature of the invention is the use of a cap on the coil holderas the surface for transmitting the vibrations 'between'the coil holderand the diaphragm.

The known electromagnetic assemblies for electroacoustic transducers,and particularly for loud speakers have to face the well known problemof the generation of excessive heat within the zone ofthe gap andtherefore, it is an important feature of the invention to be able torefrigerate said zone, whereby the usual life of the electromagneticassembly is undoubtedly increased, or in other words, the life of thedriving unit of the loudspeakers is increased. When the electromagneticassembly is used for flat loudspeakers, it is an aim that the vibrationsbe more energetic and therefore the amount of heat which is generated isstill larger. Thus, a good refrigeration within the air gap zone is evenmore important when the electromagnetic assembly is used in relationshipwith flat loudspeakers.

As a consequence of the requirement hereinabove stated, or in otherwords, to produce more energetic vibrations, it is necessary to providesuitable means in the coil holder which prevent the coil wound on thecoil holder from being removed -by shocks therefrom. Another feature ofthe present invention, and which is also. an indirect consequence of thetendency to obtain larger or more energetic vibrations is that specialmeans had to be conceived to link the ends of the coil with theconnectingconductors which link the coil with the rest of the electroniccircuit to avoid cracking or breakage at the linkage or welding pointsdue to difference in masses existing between both types of conductors,upon being subject to variable rectilinear movements generated by thevibrations of the coil holder.

Still another feature which fonns part of the present invention isrelated to the output of the loudspeakers. It is well known that in theknown loudspeakers the impedance is substantially constant in the zoneof low frequencies Hz to 5,000 Hz) and thereafter a weak growingtendency'up to the region of 20 KHz (20,000 cycles) can be seen. In theknown loudspeakers there is at least one low frequency peak. To opposethe impedance increase in the high frequency zone and to thus obtain abetter output and a substantial elimination of the peaks, according tothe tests carried out, it has been proved that it is possible toshort-circuit the coil holder at the same time as the coil is wound insuch a way as to obtain an additional capacity between the turns, forthe zones which correspond to the residual capacity between the turnsand thus obtain the already mentioned distributed capacity. There areseveral ways of short-circuiting the coil, as will be seen in thespecific examples,

The present invention, according to one of the features thereof,utilizes an electromagnetic assembly for electroacoustic transducers andthe like, comprising a magnet, a core defining a gap in said magnet, acoil holder, a coil wound on said coil holder movably arranged in saidgap and an end portion of said coil holder projecting out of saidmagnet, wherein the coil holder is provided with a seal cap in theportion projecting out of said gap, 21 space being formed between saidcap, coil holder, core and gap, at least one perforation connecting saidspace with another surrounding, thus a rectiliner to and fro moving pumpbeing defined between said cap, coil holder and core.

The electromagnetic assembly may further comprise a resilient spider,mounted on the magnetic assembly and linked to said end portion of saidcoil holder. The resilient spider or support may be an impermeable andsealing diaphragm. v

The perforation mentioned in the above paragraphs which connects saidspace with said other surrounding, may be a through perforation in thecore, so that one of the ends of said perforation faces said cap. Theair mass which is conveyed by the coil holder with its cap through theperforation, may act as a cooling means for the core, the gap, the coilholder and the coil, but it may also act as a resilient cushion,specially when the electromagnetic assembly is used under water, whereno refrigeration is necessary; in this event the other end of saidperforation is sealed.

BRIEF DESCRIPTION OF THE DRAWINGS These and other features will beexplained with more details in relationship with the specific exampleswhich will be now described by way of example in connection with thedrawings, in which:

FIG. 1 is a perspective view of an electromagnetic assembly forelectroacoustic transducers and the like, according to the presentinvention.

FIG. 2 shows in perspective view a detail of a portion of the flatdiaphragm in the zone where it is to be linked to the electromagneticassembly of FIG. 1.

FIG. 4 is a longitudinal section of the electromagnetic assembly appliedto a flat diaphragm of a flat loudspeaker.

FIG. 5 is a detail in longitudinal section, similar to FIG. 4, butshowing a possible alternative embodiment in relationship to the pumpingsystem.

FIG. 6 is a detail in longitudinal section, similar to FIG. 4, butshowing another possible alternative embodiment in relationship to thepumping system.

, FIG. 7 is a detail in longitudinal section of the coil holder, coiland resilient support.

FIG. 8 is a detail in longitudinal section, similar to FIG. 4, butshowing another possible alternative embodiment in relationship to thepumping system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIGS. 1 to4, it may be appreciated that the electromagnetic assembly 1 of thepresent invention comprises a magnet 2 which is a cylinder of smallheight and has a central cylindrical through hole 3.

A first disc 5, having a central projecting cylindrical core 6,, thediameter of which is smaller than that of ,.bore '3 and the height ofwhich is larger than that of ration 10 the diameter of which is slightlylarger than the diameter of the core 6, thus allowing between both theinsertion of a coil holder 12, having a-coil 15. The height of thecore.6 is such, when the three members 2, 5 and 9 are assembled, thatthe upper base 6 of the core 6 is flush with the upper face 25 of thesecond disc 9 (FIG. 4). Thus, an air gap 11 is formed between the core 6and the magnet 3. Gap 11 houses the coilholder '12. The coil holder 12will perform a rectilinear alternative or to and fro movement, which issubstantially coaxial with the longitudinal axis of the assembly.

The coilholder 12 is in one of the possible embodiments, a paper madecylinder 13, the inside face of which is sheathed with a metal sheet 14,for instance an aluminum foil. Although the coil holder 12 need not bean endless cylinder, but have a longitudinal cut so as to define alongitudinal slot, as is known in the art, it is preferred to make thecoil holder as an endless cylinder. On the outside face of the coilholder 12 and more particularly on the paper 13, a coil 15 is woundwhich covers the lower half, leaving free the upper half, which definesthe end .portion which projects out of the gap 11. Thus, the outputwithin the high frequency range is increased. I

To achieve the features of the distributed capacity, already aboveexplained, it is preferred to produce the coil for instance by means oftwo layers of turns, of which the inner layer of turns 16 (FIG. 7) is sowound as to progressively decrease the space between turns from bottomto top, while the outer layer of turns 17 is wound so as toprogressively decrease the space between turns from top to bottom,whereby an optimum turn density is achieved to fulfil the purpose. Oncethe coil 15 is thus wound on the coil holder 12, both the lower end rim18 as well as the upper end rim 19 are outwardly turned over thusforming each a flange, although for different purposes. The lower flangel8 prevents the coil 15 from being shock-removed from the coil holder12, when the assembly is strongly vibrating. The upper flange 19 isprovided, in order to increase the binding surface which links the upperend 19 with the resilient support which is a kind of a diaphragm 20. Inthe known electromagnetic assemblies this diaphragm or resilient support20, also called spider, is generally made of an impregnated porousfabric to convey the necessary resiliency and yet rigidity and suchknown spider may be used in connection with the present invention;however, it is also possible to replace said fabric spider by an airimpermeable member, such as of neoprene rubber.

In both cases the diaphragm 20 has an edge 21 defining the larger baseportion, from which rises a frustoconical portion 22, the smaller. baseportion 23 of which is provided with concentric ondulations ending in aconnecting edge 24 which is connected by means of an adhesive (notshown) to the upper end or flange 19 of the coil holder 12 (FIG. 7).Bearing in mind that the flange 19 is turned over, the connectingsurface between the resilient suspension 20 and the coil holder 12 isconsiderably increased. The edge 21 is connected to the upper face 25(FIG. 1) of the second disc 9 of the magnet assembly comprised by disc5, magnet 2, and disc 9. The smaller base portion 23 of diaphragm 20provides the necessary resiliency to allow for the to and fro movementof the coil holder 12.

The upper flange 19 of the coil holder 12 satisfies also anotherpurpose, namely to increase the supporting surface for the cap 26 madeof an insulating material, which is preferably soft, and which, forexample, may be made of polyethylene or polyurethane. This cap 26 has alower cylindrical porjection 27 which enters the coil holder 12 and issealed thereto at the same time as the main disc-shaped body 28, oflarger diameter than the lower cylindrical projection 27, overlaps theupper flange l9'of the coil holder 12. The body 28 and flange 19 thusdefine additional sealing surfaces. Conveniently, cap 26 is connected tothe coil holder 12 by an adhesive (not shown). It is apparent that thejunction between lower cylindrical projection 27 and the main discshaped body 28 defines a flange receiving portion which includessurfaces generally transverse and parallel to the axis of the coilholder, and this portion receives the flange 19 with the adhesive (notshown) being interposed between the opposed surfaces of the holder andthe cap. The upper face of the upper I disc-shaped body 28 has twoparallel channels 29, 29'.

The thus assembled electromagnetic assembly is connected, in case ofbeing used with a sound transducer, with a'flat diaphragm 34 which mayform part of a loudspeaker or a microphone. This flat diaphragm may forinstance be of the type described in my US. Pat. No. 3,596,733, or anyother type of known flat loudspeaker, as cited in the introductoryportion of said patent. Diaphragm 34 corresponds to a flat loudspeakerhaving a front face 35 and a rear face 36. This rear face 36 has acircular recess 37, from which extends a pair of parallel channels 38,38. The circular recess 37 is filled with an adhesive (not shown) andthe cap 26 is glued through its top face 26 thereto so that the portionof theconductors 31, 31 and the insulated conductors 32, 32' projectingaway from cap 26 become housed in the channels 38, 38, thus building ahomogeneous assembly and further reducing the possibility of breakage ofthe conductors. Obviously, conductors 32, 32 are furthermore connected,as already stated, to a pertinent electronic circuit (not shown). Incase the top face 26 is not perfectly parallel to the bottom wall of therecess 37, the adhesive which links the two members compensate any lackof parallelism and the homogeneous assembly is thus formed. As to theadhesive, the latter may be a hard adhesive, or in other words itproduces a rigid connection between the recess 37 and the top 26'. It isalso possible to use a soft adhesive, such as a resilient adhesive,whereby the intermodulation is improved.

Finally, if the assembly of coil 15, coil holder 12 and cap 26 operatesas a percussion member with relationship to the diaphragm 34, noadhesive will be required. Whether or not the cap is secured to thediaphragm, the cap'is a member independent of the diaphragm and definesa large contact top surface directed towards the diaphragm and indriving relationship with the diaphragm, so that, when used as amicrophone, for example, the diaphragm drives the cap through saidsurface, and when used as a speaker, for example, the cap drives thediaphragm through said surface.

If it is desired to connect the electromagnetic assembly 1 hereinabovedescribed to an orthodox frustoconical loudspeaker, this is alsopossible and therefore the present invention has not to be interpretedin a limiting manner in this direction.

As to the operation of the structural embodiment described, it isobvious that in principle it is the same as the one of a knownelectromagnetic assembly as far as the generation of the electromotiveforce for vibrating the coil holder 12 is concerned. The air which ishoused between the lower cylindrical projection 27 of the cap 26 and thecore 6, is a mass of air which is pumped towards the inside and towardsthe outside through perforation 7, thereby achieving a refrigeratingeffect.

It would also be possible to replace perforation 7 of FIG. 4 byperforations 7'(FIG. 8) which are present in the first disc 5surrounding the core 6, so that the movement of refrigerating or coolingair is performed through the gap 11. i

If the diaphragm is air permeable, the assembly can likewise operate ina satisfactory manner.

When the assembly is to be used below the water, perforation 7 is sealed(FIG. 5) by means of a plug rod 39. During the assembling of theelectromagnetic assembly, the position of rod 39 is gauged with regardto perforation 7 in order to achieve good operation. In this event, noresilient spider suspension of the type of diaphragm 20 is necessarysince the air cushion can carry out this role, but upon using theelectromagnetic assembly under water, sealing means must be provided inorder to avoid that the water enters the gap 11.

With regard to the embodiment shown in FIG. 6, here the plug rod 39 isreplaced by a tubular member 40, having a valve 41 for gauging acrosssectional passage for the air, so that an optimum relationship canbe achieved. Obviously, valve 41 may be replaced by a tubular memberwith a pre-established cross-sectional area for the air passage. In thesame example which defines an alternative embodiment, a furthermodification is shown which as such is applicable to any of the otherstructural embodiments.

When the electromagnetic assembly of the present invention is used witha loudspeaker, for instance of the flat type as described inrelationship to FIG. 2, it may be that special effects are intended tobe achieved, such as an increased range of low frequencies. In otherwords, in a musical piece it may be desired that the human voice becomessubmerged within the musical poem. To achieve this effect in the cap 26,and more particularly in the lower projection 27 a recess 42 may be madeto house therein a weight member 43. Thus, the mass of the movableassembly and more particularly of the cap 26 and coil holder 12' withits coil 15 is increased, whereby the assembly will operate within alarger range of low frequencies.

The coil holder 12 described in relationship with FIG. 7 has arelatively low mechanical resistance and it may be convenient that suchresistance is increased. In the embodiment shown in FIG. 6, the coilholder 12 may for instance be made of plastified (bakelized) cardboard,whereby a larger resistance against impacts is achieved upon the coilholder entering in impacting contact with the first disc 5. A coilholder so made, has i the drawback .that it is inefficiently cooled, butthis drawback can be compensated by producing the weight member 43 of agood electricity and heat conducting material, such as copper, and sincesaid copper disc or plate 43 faces the mass of air which is pumpedthrough the perforation 7, the copper disc transfers the heat which itabsorbs to the cooling air mass. If desired, a heat insulating layer 44(FIG. 6) is arranged between the metal disc 43 and said cap 26.

If the cap 26 is made of expanded, bonded, cellular plastics, such aspolystyrene, it may become advisable to arrange a heat insulating massbetween the plate 43 and the recess 42, such as an asbestos disc-member(not shown) to avoid that the cap 26 becomes injured by heat.

Another possibility would be to manufacture the cap 26 from a rigidplastic, such as nylon.

It will be obvious that modifications may be introduced in thisinvention upon carrying it out into practice as far as certainstructural details are concerned but always without departing from thefundamental principles which are clearly specified in the followingclaims.

I claim:

1. An electroacoustic transducer comprising a diaphragm and anelectromagnetic assembly in driving relationship with said diaphragm,said electromagnetic assembly comprising a magnet, a core defining a gapin said magnet, a coil supported by a coil holder movably arranged insaid gap with an end portion of said coil holder facing said diaphragm,an independent cap on 32. An electromagnetic assembly forelectroacoustic transducers and the like, comprising a magnet, a coredefining a gap in said magnet, a coil holder having a coil woundthereon, said coil holder being movably arranged in said gap and an endportion of said coil holder projecting out of said magnet, said coilholder being provided with a seal cap in the portion projecting out ofsaid gap, a space being formed between said cap, coil holder, core andgap, at least one perforation connecting said space with anothersurrounding, whereby a rectilinear to and fro moving pump is defined bysaid cap, coil holder and core, said coil holder being an endlesscylinder the outer face of which is of electricity insulating materialand the inner face of which is sheathed with an electricity conductinglayer.

33. An electromagnetic assembly for electroacoustic transducers and thelike, comprising a magnet, a core defining a gap in said magnet, a coilholder having a coil wound thereon, said coil holder being movablyarranged in said gap and an end portion of said coil holder projectingout of said magnet, said coil holder being provided with a seal cap inthe portion projecting out of said gap, a space being formed betweensaid cap, coil holder, core and gap, at least one perforation connectingsaid space with another surrounding, whereby a rectilinear to and fromoving pump is defined by said cap, coil holder and core, said caphaving an upper face remote from said coil holder, said cap beingprovided on its upper face with a pair of channel members in whichrespective end portions of the conductor of the coil are housed andconnected to respective other conductors. I I

34. An electromagnetic assembly according to claim 52, wherein said capis of soft plastic.

35. An electromagnetic assembly for electroacoustic transducers and thelike, comprising a magnet, a core defining a gap in said magnet, a coilholder having a coil wound thereon, said coil holder being movablyarranged in said gap and an end portion of said coil holder projectingout of said magnet, said coil holder being provided with a seal cap inthe portion projecting out of said gap, a space being formed betweensaid cap, coil holder, core and gap, at least one perforation connectingsaid space with another surrounding, whereby a rectilinear to and fromoving pump is defined by said cap, coil holder and core, said coreholder being made of plastified cardboard and said cap housing a goodheat and electricity conducting metal disc facing said space.

. 36. An electromagnetic assembly according to claim 35, wherein a heatinsulating layer is arranged between said metal disc and said cap.

37. An electromagnetic assembly according to claim 33, wherein saidcap-is made of hard material.

1. An electroacoustic transducer comprising a diaphragm and anelectromagnetic assembly in driving relationship with said diaphragm,said electromagnetic assembly comprising a magnet, a core defining a gapin said magnet, a coil supported by a coil holder movably arranged insaid gap with an end portion of said coil holder facing said diaphragm,an independent cap on said end portion defining a large contact topsurface directed towards, and in driving relationship with, saiddiaphragm.
 2. An electroacoustic transducer according to claim 1,wherein said surface overlies said holder.
 3. An electroacoustictransducer according to claim 2, wherein said surface is secured to saiddiaphragm.
 4. An electroacoustic transducer according to claim 3,wherein substantially the entire top surface of said cap is secured tosaid diaphragm.
 5. An electroacoustic transducer according to claim 4,wherein said surface is adhesively secured to said diaphragm.
 6. Anelectroacoustic transducer according to claim 4, wherein said surface isadhesively secured to said diaphragm by means of an adhesive interposedbetween said surface and said diaphragm.
 7. An electroacoustictransducer according to claim 6, wherein said adhesive is a hardadhesive to provide a rigid connection between said surface and saiddiaphragm.
 8. An electroacoustic transducer according to claim 6,wherein said adhesive is a soft adhesive to provide a resilientconnection between said surface and said diaphragm.
 9. Anelectroacoustic transducer according to claim 1, wherein said diaphragmis a flat diaphragm.
 10. An electroacoustic transducer according toclaim 9, wherein said surface overlies said holder.
 11. Anelectroacoustic transducer according to claim 10, wherein said surfaceis secured to said diaphragm.
 12. An electroacoustic transduceraccording to claim 11, wherein substantially the entire top surface ofsaid cap is secured to said diaphragm.
 13. An electroacoustic transduceraccording to claim 12, wherein said surface is adhesively secured tosaid diaphragm.
 14. An electroaCoustic transducer according to claim 12,wherein said surface is adhesively secured to said diaphragm by means ofan adhesive interposed between said surface and said diaphragm.
 15. Anelectroacoustic transducer according to claim 14, wherein said adhesiveis a hard adhesive to provide a rigid connection between said surfaceand said diaphragm.
 16. An electroacoustic transducer according to claim14, wherein said adhesive is a soft adhesive to provide a resilientconnection between said surface and said diaphragm.
 17. Anelectroacoustic transducer according to claim 15, wherein said diaphragmhas a recess which substantially houses said adhesive.
 18. Anelectroacoustic transducer according to claim 16, wherein said diaphragmhas a recess which substantially houses said adhesive.
 19. Anelectroacoustic transducer according to claim 1, wherein said cap has aflange receiving portion including surfaces generally transverse andparallel to the axis of said coil holder, said end portion of said coilholder having laterally projecting flange means received in said flangereceiving portion and wherein an adhesive is interposed between theopposed surfaces of said holder and said cap.
 20. An electroacoustictransducer according to claim 1, wherein said coil holder has flangemeans extending laterally below said coil to prevent the coil fromsliding off said coil holder.
 21. An electroacoustic transduceraccording to claim 1, wherein said cap seals said end portion of saidcoil holder, a space being formed between said cap, coil holder, coreand gap, at least one perforation connecting said space to anothersurrounding, whereby a rectilinear to and fro moving pump is defined bysaid cap, coil holder and core.
 22. An electroacoustic transduceraccording to claim 21, wherein the upper end of the coil holder issealed to a frustoconical air impermeable resilient support having alower flange which is sealed to the electromagnetic assembly.
 23. Anelectroacoustic transducer according to claim 21, wherein saidperforation is a through perforation passing through said core, one endof said perforation facing said cap.
 24. An electroacoustic transduceraccording to claim 23, wherein said through perforation is sealed at itslower end.
 25. An electroacoustic transducer according to claim 23,wherein said perforation is provided with a valve member for changingthe cross sectional area passage thereof.
 26. An electroacoustictransducer according to claim 1, wherein said cap has an upper faceremote from said coil holder, said cap is provided on its upper facewith a pair of channel members in which respective end portions of theconductor of the coil are housed and connected to respective otherconductors.
 27. An electroacoustic transducer according to claim 1,wherein said cap is constituted by a soft plastic.
 28. Anelectroacoustic transducer according to claim 1, wherein said cap isconstituted by a hard plastic.
 29. An electroacoustic transduceraccording to claim 14, wherein said cap has an upper face remote fromsaid coil holder, said cap is provided on its upper face with a pair ofchannel members in which respective end portions of the conductor of thecoil are housed and connected to respective other conductors, saidadhesive substantially covering said channels and conductors andconnections therein.
 30. An electroacoustic transducer according toclaim 21, wherein said core forms part of a disc abutting on saidmagnet, said disc having a plurality of perforations connecting theoutside with said gap.
 31. An electromagnetic assembly forelectroacoustic transducers and the like, comprising a magnet, a coredefining a gap in said magnet, a coil holder having a coil woundthereon, said coil holder being movably arranged in said gap and an endportion of said coil holder projecting out of said magnet, said coilholder being provided with a seal cap in the portion projecting out ofsaid gap, a space being formed between said cap, coil holder, cOre andgap, at least one perforation connecting said space with anothersurrounding, whereby a rectilinear to and fro moving pump is defined bysaid cap, coil holder and core, said coil holder having an upper end anda lower end, said coil being wound on the lower half of said coil holderby means of two juxtaposed layers of turns, the spacing between adjacentturns of one of said layers progressively increasing toward the upperend and the spacing between adjacent turns of the other of said layersprogressively increasing toward the lower end.
 32. An electromagneticassembly for electroacoustic transducers and the like, comprising amagnet, a core defining a gap in said magnet, a coil holder having acoil wound thereon, said coil holder being movably arranged in said gapand an end portion of said coil holder projecting out of said magnet,said coil holder being provided with a seal cap in the portionprojecting out of said gap, a space being formed between said cap, coilholder, core and gap, at least one perforation connecting said spacewith another surrounding, whereby a rectilinear to and fro moving pumpis defined by said cap, coil holder and core, said coil holder being anendless cylinder the outer face of which is of electricity insulatingmaterial and the inner face of which is sheathed with an electricityconducting layer.
 33. An electromagnetic assembly for electroacoustictransducers and the like, comprising a magnet, a core defining a gap insaid magnet, a coil holder having a coil wound thereon, said coil holderbeing movably arranged in said gap and an end portion of said coilholder projecting out of said magnet, said coil holder being providedwith a seal cap in the portion projecting out of said gap, a space beingformed between said cap, coil holder, core and gap, at least oneperforation connecting said space with another surrounding, whereby arectilinear to and fro moving pump is defined by said cap, coil holderand core, said cap having an upper face remote from said coil holder,said cap being provided on its upper face with a pair of channel membersin which respective end portions of the conductor of the coil are housedand connected to respective other conductors.
 34. An electromagneticassembly according to claim 52, wherein said cap is of soft plastic. 35.An electromagnetic assembly for electroacoustic transducers and thelike, comprising a magnet, a core defining a gap in said magnet, a coilholder having a coil wound thereon, said coil holder being movablyarranged in said gap and an end portion of said coil holder projectingout of said magnet, said coil holder being provided with a seal cap inthe portion projecting out of said gap, a space being formed betweensaid cap, coil holder, core and gap, at least one perforation connectingsaid space with another surrounding, whereby a rectilinear to and fromoving pump is defined by said cap, coil holder and core, said coreholder being made of plastified cardboard and said cap housing a goodheat and electricity conducting metal disc facing said space.
 36. Anelectromagnetic assembly according to claim 35, wherein a heatinsulating layer is arranged between said metal disc and said cap. 37.An electromagnetic assembly according to claim 33, wherein said cap ismade of hard material.